Analyzing chemical compositions of samples is important in many contexts, including identifying and segregating metal types (particularly various alloys of iron and steel) in outdoor metal recycling facilities, quality control testing in factories and forensic work. Several analytical methods are available.
Optical emission spectroscopy (OES) is a mature, robust technology for the elemental analysis of materials. In OES, a small quantity of sample material is vaporized and excited above atomic ground state. Emissions characteristic of elements in the vaporized sample are captured by a light guide, which sends the light to a spectrometer, which produces and analyzes a spectrum from the light, so as to yield information about the elemental composition.
For electrically conductive samples, prevalent techniques for generating emission spectra use either an electric arc or a spark, or both, to vaporize a small quantity of the sample to be analyzed. An electrical potential in an analytical gap between a counterelectrode and a surface of the sample breaks down gas in the gap, enabling an electrical current, in the form of a spark or an arc or both, to flow between counterelectrode and the sample surface. Typically, the spark or arc vaporizes a portion of the sample and causes the vaporized sample to move into the analytical gap, and thereafter heats the gas in the gap, thereby exciting the vaporized sample material. In the resulting plasma, the excited sample (“analyte”) produces an optical (although possibly invisible) discharge that is characteristic of the elemental composition of the excited material.
Alternatively, laser-induced breakdown spectroscopy (LIBS) or glow discharge (GD) may be used to vaporize and excite an emission sample. A survey of OES analytical techniques may be found in K. Slickers, “Automatic Atomic-Emission Spectroscopy”, Second Edition (1993), which is incorporated by reference as if fully set forth herein for all purposes.
Regardless which excitation technique is used, an image of the excited sample is projected onto an entrance slit of a spectrometer, which analyzes composition of the sample, based on wavelengths and intensities of the optical signal. Emissions from the analyte should be sampled from a volume of the analytical gap where the analyte is ionized. Optical signals from other sources, such as the heated tip of the counterelectrode or the sample surface, could confuse the analysis and should not, therefore, be allowed to enter the spectrometer.
A mask and/or a suitably short slit may be used to exclude these unwanted emissions. However, masks and short slits limit the amount of optical signal received by the spectrometer, leading to poor signal-to-noise ratios.